This proposal continues a long-standing collaboration between the laboratories of Judah Folkman and Douglas Hanahan that has focussed on the mechanisms of tumor angiogenesis and therapeutic applications of such new knowledge. The long-term objectives are to identify the critical components of the regulatory machinery that is abrogated to switch angiogenesis on and to sustain it. The progress from this joint program has centrally implicated endogenous angiogenesis inhibitors in this misregulation, presenting both therapeutic and mechanistic opportunities. An emerging paradigm is that proteases are involved in many aspects of angiogenic regulation, ranging from excision of inhibitors embedded in larger proteins, to release of sequestered angiogenic growth factors. The two laboratories have synergistic, complementary expertise in protein biochemistry and endothelial cell biology and in molecular genetics and transgenic mouse models of carcinogenesis. The specific aims are: 1. Develop a new organizing principle for discovery of endogenous angiogenesis inhibitors that exist as internal fragments of hemostatic proteins. 2. Determine the role of an exciting class of intercellular signaling molecules, the ephrins in the angiogenic switch and in specification of tumor vasculature. 3. Genetically define the key regulatory genes among candidate activators, inhibitors and metalloproteinases, for the angiogenic switch during multistage tumorigenesis in transgenic mouse models of endocrine, pancreatic, cutaneous and cervical carcinoma. 4. Further develop experimental therapeutic approaches that utilize angiogenesis inhibitors for treatment or prevention of endogenous cancers in transgenic mouse models.